Application of the dianion croconate violet for symmetric organic non-aqueous redox flow battery electrolytes

Redox active organic molecules (ROMs) are promising candidates for redox flow battery (RFB) energy storage due to their high sustainability and low cost. Herein, the pseudooxocarbon derivative croconate violet (Croc2-) is applied as a novel symmetric ROM in acetonitrile electrolyte, whereby Croc2- is used as both the battery posolyte and negolyte, with a 1.82 V cell potential and ≃1 M solubility. Characterisation of the dianion Croc2- is given by way of voltammetry and battery cycling techniques to demonstrate the high number of oxidation states accessible by Croc2-, thus highlighting a high intrinsic capacity for a low molecular weight ROM. The stability of Croc2- and its charged radical states is investigated to assess the viability of the symmetric design, and an undesirable radical disproportionation mechanism of the Croc•3- oxidation state is identified to account for poor capacity retention. Asymmetric battery experiments of a Croc2- posolyte with 2,1,3-benzothiadiazole or tetracyanoquinodimethane negolytes gave improved battery performance, indicating that Croc2- is a promising anionic ROM posolyte. [Display omitted] •Redox-active organic molecules are promising new energy storage materials.•The dianion croconate violet is applied in non-aqueous electrolyte.•The performance in symmetric and asymmetric batteries is investigated.•The stability of the charged molecular radical states is assessed.•A radical disproportionation mechanism is identified to account for capacity loss.